Centuries ago people imagined Earth to be ground zero for the universe—its center and starting point—until astronomers told them otherwise. In some ways we still hold on to that kind of thinking: We measure galaxies and stars as if we were at the beginning of a billions-of-light-years-long ruler. But for most other measurements, we have had to calibrate the cosmos not by using ourselves as zero but by discovering zero points in nature or creating them from scratch. Sometimes those empty zeros prove full of value for understanding the world around us.
Take, for instance, the grid that defines locations on our planet. The zero line of longitude was arbitrarily chosen in 1884 to pass through Greenwich, England. Zero latitude is another story: That's the naturally defined equator, where residents forever experience 12 hours of day and 12 of night. When astronomers needed similar reference lines, they called an imaginary line in space above the equator zero declination and demarcated everything from there. Equidistant from the North Star and a patch of bare sky at the South Pole, this line is marked by Orion's Belt (above). For longitude's equivalent, a system of right ascension lines are inscribed on sky charts; these show what's on the meridian at hourly intervals. The zero hour is the sun's position at the vernal equinox.
Looking up toward the coldness of space, we're reminded of the zeros in the temperature scales. Daniel Gabriel Fahrenheit's zero is said to be the lowest temperature he could muster in his 18th-century lab by mixing salt water and ice—hardly a natural marker. By contrast, zero degrees Celsius is the point at which water changes from liquid to solid. Most sensible of all is zero kelvins, equal to –459.67°F and known as absolute zero. Here atomic motion (excluding quantum jittering) grinds to a halt. Scientists have approached within billionths of a degree of this temperature, blurring atoms to form a frictionless state of matter called the Bose-Einstein condensate. Absolute zero is fertile territory where materials develop light-stopping or gravity-defying properties. But we may never reach this zero. It requires a state in which no atom can be moved by any other atom—impossible according to our laws of physics, which say that everything must affect everything else in some way.
That a zero is unattainable does not detract from its value. Physicists build calculations around a fixed point of zero motion in space—even though this point is artificial, since locations in time and space are relative. Zero motion may be unattainable, but accelerate to the speed of light and we find an everyday zero: zero rest mass, a property of photons. If they could stop moving, they would, impossibly, possess no energy or mass.
We might imagine that a zero-volume object is also an impossibility, yet physicists predict that the singularity at the heart of a black hole has just that property. Our science fails in such conditions, and nobody knows what this might mean. Perhaps this zero is another starting point that will allow us to quantify things we have yet to understand.